The present invention relates to methods and apparatuses for recording and playing back video data, more particularly to a method and an apparatus for recording and playing back monitored video data, which will be suitable for recording video data for a long time for the purpose of monitoring and preventing of crimes.
There is provided a prior art time lapse VTR used as a video data recording device for recording video data from, a video camera, suitable for monitoring and preventing crimes. Many of this type VTR are improved from a home VHS VTR which is very popular at present to enable a long time recording. For example, in such a time lapse VTR, the magnetic tape can be fed at a ⅓ speed of the VHS Extended Play (EP) mode to record only one of the input video 3 fields on the tape, so that the recording time can be extended to 3 times that of the EP mode. Consequently, when a 120-min tape is used, the recording time is extended to 6 hours (120 min×3) in the VHS EP mode. The recording time of the above time lapse VTR can thus be extended to 18 hours, 3 times that of the VHS EP mode. In this case, however, as understood clearly from the recording principle explained above, the number of fields per second is 20, which is ⅓ of the VHS (60 fields). In addition, audio signals can be recorded on audio tracks arranged in the longitudinal direction of the tape just like an ordinary VTR, but the tape speed becomes ⅓ of the normal one. As a result, the sound quality is degraded more than the normal one. In spite of this, the time lapse VTR can have an advantage of 18-hour audio recording time.
There is also proposed an intermittent recording system for the above prior art time lapse VTR, in which system, the average tape speed is delayed and the number of recording fields is reduced more to extend the recording time. This system can have the maximum recording time of, for example 720 hours using a normal 120-min tape by, recording video data at a rate of 1 field per 2 seconds (120 fields) per frame (2 hours×3 times×120=720 hours). In such a super-long time recording operation, the average tape feed speed is very slow, so that it becomes difficult to control the continuous feeding of the tape. To avoid this problem, the tape is fed intermittently in most cases and accordingly, it becomes impossible to record audio signals in audio tracks.
The conventional time lapse VTR ware opened in, for example, Examined Published Japanese Patent Application No.3-12380, Examined Published Japanese Patent Application No.7-63182, etc. The former patent report describes a method for controlling tape feeding so as to obtain the same recording tape pattern as that of the ordinary VTR even at the recording time by feeding the tape intermittently just like in the above-mentioned 720-hour recording. The latter patent report (No.7-63182) describes a method for controlling the rotation speed of the cylinder to prevent generation of skew during the intermittent recording.
As explained above, the time lapse VTR needs recording modes for continuous low speed tape feeding and intermittent tape feeding in addition to the normal continuous tape feeding, and a means for precision controlling of the capstan and the cylinder corresponding to those playback modes. In a recording operation, the same playback mode as each of those recording modes is needed to check pictures at the same speed as that of the recording. For this purpose, each of those prior art VTRs must be provided with various precision control means, so their prices become more higher than general home VTRs.
The time lapse art VTR cannot load (wind a magnetic tape onto the cylinder to which a magnetic head is attached to prepare for recording or playing back) a magnetic tape (video data recording medium) and unload (separate the tape from the cylinder) the magnetic tape within a short time even for a recording operation in a recording mode for intermittent tape feeding. Therefore, the tape is needed to be kept loaded during recording. Further, since the cylinder has a large moment and takes much time until its rotation speed is stabilized, the cylinder must be rotated almost at a fixed speed continuously during the recording.
Therefore, the time lapse VTR must be kept loaded with a tape during recording of monitored video data and the cylinder is rotated continuously. Especially, in the above super-long recording mode (720-hour mode), the VTR has a disadvantage that because the cylinder is kept rotated in contact with the tape for 720 hours, the tape is damaged far more rapidly than used for a home VTR. In the same way, because the head is in contact with the tape, the head is also worn rapidly. The mechanisms such as the motor and belt used for making the cylinder rotate are also degraded rapidly and often damaged, since they are running for a long time respectively.
Further, conventional helical scanning VTR also has a following defect; the recording track pattern angle on the tape loaded is decided by both the rotation speed of the cylinder and the feed speed of the tape, so that the track pattern recorded at the above slow speed tape feeding or the tape stops has an angle different from the normal speed tape feeding. Therefore, when playing back the track pattern recorded in such way, a mismatching occurs between the tracing angle of the head and the angle of the track pattern, causing the SIN (signal-noise ratio) to be degraded in a part of the playback signals, with the resultant of lowering the quality of output pictures significantly.